Rivets are often used in the aerospace industry to securely fasten the metal or composite components of aerospace vehicles to one another. In the interest of weight conservation and strength requirements, the rivets are often constructed of high strength aluminum, titanium, or alloys thereof.
To fasten two metal components with a rivet, holes are first defined in the respective components and the holes are aligned with one another. Once aligned, a rivet is placed through the hole. The rivet comprises a head portion, which has a larger diameter than the hole and therefore does not enter the hole, and a body portion, which extends through the hole and projects from the opposing side of the joined components. To lock the rivet in place, a force is applied to the region of the rivet body that is opposite the head and projecting past the joined components, while the head is held in place. This force deforms the body and provides a structural junction between the two components. The act of deforming the rivet is known as upsetting.
After installation, the rivet is expected to keep the metal components joined together over the life of the aerospace vehicle. Due to the nature of aircraft and other aerospace vehicles, the rivets may be exposed to great physical stresses. In addition, the rivets must endure various environments, wet and dry, hot and cold, depending upon the exposure of the vehicle to various conditions. As a result of these stresses and environmental conditions, rivets sometimes crack or otherwise fail.
Experience indicates that grain size of the material from which the rivet is constructed is a primary consideration in increasing rivet longevity and decreasing rivet failure rates. Rivets of material having fine grain sizes have been shown to be less prone to cracking upon upsetting and are also less prone to long-term stress corrosion cracking.
Precipitation hardening heat treatment is currently the only method to produce high strength aluminum alloy rivets having fine grain sizes. Rivets made from precipitation heat treated aluminum alloys remain adequate for most modern day applications but fall short of today's high-performance needs.
There are problems related to precipitation heat treatment microstructures, including cracking during rivet head upsetting due to a deformation mechanism known as strain localization caused by strain softening on shear planes as the coherent hardening precipitates become sheared by deformation dislocations. Another problem with precipitation heat treatment microstructures is cracking during service due to residual stresses from the water quench and rivet upsetting and to the well known susceptibility of these precipitation hardened microstructures to stress corrosion cracking. Further, precipitation heat treatment introduces residual stress and distortion in the metal, which may lead to eventual failure of the rivet.
What are needed are improved aluminum alloy rivets that do not depend upon precipitation heat treating techniques and that are capable of withstanding the extreme temperatures, corrosive environments, and extreme mechanical stresses inherent in high-performance aerospace vehicles. To achieve these goals, it is desired to produce an aluminum alloy rivet having fine grain size, extremely fine and thermally stable (non-heat treatable) dispersoid particles, and a matrix alloy composition which resists strain localization.